The retina is a layer of specialized light sensitive neural tissue located at the inner surface of the eye of vertebrates. Light reaching the retina after passing the cornea, the lens and the vitreous humor is transformed into chemical and electrical events that trigger nerve impulses. The cells that are responsible for transduction, the process for converting light into these biological processes are specialized neurons called photoreceptor cells.
The retinal pigment epithelium (RPE) is a polarized monolayer of densely packed hexagonal cells in the mammalian eye that separates the neural retina from the choroid. The cells in the RPE contain pigment granules and perform a crucial role in retinal physiology by forming a blood-retinal barrier and closely interacting with photoreceptors to maintain visual function by absorbing the light energy focused by the lens on the retina. These cells also transport ions, water, and metabolic end products from the subretinal space to the blood and take up nutrients such as glucose, retinol, and fatty acids from the blood and deliver these nutrients to photoreceptors.
RPE cells are also part of the visual cycle of retinal: Since photoreceptors are unable to reisomerize all-trans-retinal, which is formed after photon absorption, back into 11-cis-retinal, retinal is transported to the RPE where it is reisomerized to 11-cis-retinal and transported back to the photoreceptors.
Many ophthalmic diseases, such as (age-related) macular degeneration, macular dystrophies such as Stargardt's and Stargardt's-like disease, Best disease (vitelliform macular dystrophy), and adult vitelliform dystrophy or subtypes of retinitis pigmentosa, are associated with a degeneration or deterioration of the retina itself or of the RPE. It has been demonstrated in animal models that photoreceptor rescue and preservation of visual function could be achieved by subretinal transplantation of RPE cells (Coffey et al. Nat. Neurosci. 2002:5, 53-56; Lin et al. Curr. Eye Res. 1996:15, 1069-1077; Little et al. Invest. Ophthalmol. Vis. Sci. 1996:37, 204-211; Sauve et al. Neuroscience 2002:114, 389-401). There is a need to find ways to produce RPE cells, such as from human stem cells, that can be used for the treatment of retinal degenerative diseases and injuries.